1. Field of the Invention
The present invention generally relates to a new cryostat system and, more particularly, to a cryostat system for cooling a surface to which a device to be cooled is attachable, with superfluid helium, without the need to store superfluid helium.
2. Description of the Prior Art
There are many experiments, including some performed in space explorations in which cooling of devices down to very low temperatures on the order of 2.degree.K or less is required for relatively long periods of time. For example, a spacecraft mission for mapping the celestial sphere in the far-infrared has been proposed. For such a mission, the infrared (IR) detector has to be maintained at about 2.degree.K or less for long periods of time, on the order of six months. The cooling medium, generally referred to as the cryogen, which comes to mind for such an application is superfluid helium, often designated as HeII. As is known helium is present in the superfluid state at the temperature of its .lambda.-point which is 2.18.degree.K and below. Thus, from a temperature point of view HeII is a very desirable cryogen for temperatures of 2.18.degree.K and less, such as 2.degree.K or less.
One well known property of superfluid helium is that it flows through the smallest crack or flaw in metal structures, making its storage for use over extended periods most difficult. Also, very little is known about its behavior in the zero gravity environment of space. It is however believed that the problem of storing it in large quantities in space will be compounded by its superfluidity. For space exploration the superfluid helium will have to be stored in a tank generally referred to as a dewar which will be surrounded by a vacuum shield to minimize the increase in the helium temperature due to heat radiation. The large quantity of superfluid helium would tend to slosh around in the dewar thereby making accurate attitude controls of the spacecraft most difficult. Also the superfluid helium would tend to flow through the smallest flaws in the dewar into the vacuum shield thereby affecting the vacuum which will in turn reduce the shielding of the dewar from radiated heat.
Thus, a need exists for a cryostat system to cool one or more objects to very low temperatures on the order of 2.degree.K or less without having to store a large quantity of superfluid helium. A further desirable feature of the cryostat system is to be able to store a large quantity of helium, in other than the superfluid state, to cool different objects located at different locations on a spacecraft to the same or different controllable temperatures on the order of 2.degree.K or less over an extended period of time and to make maximum efficient use of the helium for other than object-cooling purposes.